An injection molded product which is injection-molded using resins such as polyolefin resins, polystyrene resins, ABS resins, polycarbonate resins, and polyamide resins has been widely used as a resin product. For example, in automobile interior parts, an injection molded product which is provided with a rib or boss for providing rigidity, structural strength, or the like, or a clip and the like (hereinafter, referred to as “rib and the like”) for attachment, on a non-design surface (rear surface) of a plate-like portion, has been used.
Such an injection molded product can be obtained by, for example, molding and forming a resin by injection-filling a mold cavity, which is formed by mold-clamping a design surface mold forming a design surface side of the injection molded product and a non-design surface mold forming a non-design surface side. In the molding of the injection molded product, in some cases, a sink mark is generated on a surface of the obtained injection molded product caused by volume shrinkage of the resin which is injection-filled in the mold cavity. Particularly, in the case of the above-described injection molded product provided with the rib and the like on a non-design surface side of a plate-like portion, the resin of the portion coming into contact with the rib and the like on the plate-like portion is easily drawn to the side of the rib and the like during molding, and thus, sink marks are easily generated on a design surface of the portion.
Examples of the method of controlling the generation of sink marks on the design surface of the injection molded product includes the following methods (i) to (iv).
(i) A method of performing injection molding in a state in which the temperature of a cavity-forming surface of a mold is maintained at greater than or equal to a glass transition temperature of a resin (Patent Document 1).
(ii) A method of injection-filling the inside of a mold cavity with a resin, subsequently injecting gas into the mold cavity from a non-design surface mold side, and molding the resin by pressing the resin into the cavity-forming surface of a design surface mold using the gas pressure (Patent Document 2).
(iii) A method of injection-filling a mold cavity of a mold, which is heated to a temperature higher than or equal to a glass transition temperature or higher than or equal to a thermal deformation temperature of a resin, with the resin, and subsequently lowering the temperature of the mold of a design surface mold and subsequently a non-design surface mold to a temperature lower than or equal to the glass transition temperature or lower than or equal to the thermal deformation temperature, and subsequently lowering the temperature of the mold a non-design surface mold to the temperature lower than or equal to the glass transition temperature or lower than or equal to the thermal deformation temperature (Patent Document 3).
(iv) A method of performing injection-molding using a mold in which a heat insulation layer is formed on a cavity-forming surface of a design surface mold and a surface metal layer is further formed on the heat insulation layer (Patent Document 4).
Meanwhile, in automobile interior parts, a crystalline polypropylene resin is used for the injection molding in many cases. The crystalline polypropylene resin shrinks volume greatly due to crystallization. For this reason, sink marks are easily generated in an injection molded product using the crystalline polypropylene resin compared to an injection molded product using other resins. For example, as to the method (i), molding of the polypropylene resin is generally performed at about 40° C. of a surface temperature of a mold. In addition, it has been known that the glass transition temperature of the polypropylene resin is −20° C. That is, the general molding condition of the polypropylene resin meets the condition disclosed in Patent Document 1, but it is a well-known fact that it is impossible to suppress sink marks formed during molding even under such a condition.
Similarly, even in the methods (ii) and (iv), it is impossible to sufficiently suppress the generation of sink marks on the design surface when molding the polypropylene resin.
In addition, in fields such as automobile interior parts, reduction of the thickness of an injection molded product has been carried out in view of weight reduction or energy conservation. When the thickness of the injection molded product provided with a rib and the like on the non-design surface side of the plate-like portion is reduced, the thickness ratio of the portion which is provided with the rib and the like to the portion which is not provided with the rib and the like in the injection molded product becomes large. Accordingly, the resin forming the plate-like portion is drawn to the rib and the like during the molding, thereby promoting generation of noticeable sink marks on the design surface of the portion. Therefore, it is more difficult to perform the suppression of sink marks.
From the above, in the injection molded product using the crystalline polypropylene resin, the flexibility of design of the product has been impaired when reducing the thickness of the product for the purpose of weight reduction.
In addition, in the method (iii), there is a problem in that energy loss is extremely great since heating and cooling of the mold is repeated while continuously manufacturing the injection molded product. In addition, it is necessary to use a special mold in the method (iv), which is disadvantageous even in view of maintenance of the mold.